Most microwave electron accelerators have the capability of varying pulse beam current by changing the amount of current injected into the accelerator structure. However, each accelerator structure or component which supplies current to the structure, has a limited dynamic range over which it will operate efficiently.
Decelerators, made from low-atomic number materials such as carbon, can be used to reduce the transmitted beam current as well as the energy of the accelerated electrons. However, in some cases it is required to reduce only the beam current while maintaining the high initial electron energy. In addition, the low-atomic number materials will induce electron energy straggling which will adversely effect the surface dose distribution for some applications.
One or more scattering foils can be made from high-atomic number materials such as tantalum or lead. However, high-atomic number materials produce x-rays efficiently which is unacceptable for some applications. Also, the dose rate from the electrons can be reduced as the reference point is moved away from the scattering foil but this may take separation distances that are unpractically large. The beam current can be reduced by using multiple scattering foils, but the total thickness of the foils required to reduce the beam current for a particular application may reduce the electron energy by an unacceptable amount.
If there is some energy spread in the accelerated electron beam, momentum analysis may be used to remove a portion of the beam current. The momentum analyzer typically takes the form of a magnet with slits that define a specific trajectory in the magnetic field, hence a specific energy. For some applications, the cost, complexity and size of the momentum analyzing system is not acceptable.